The present invention is a self-assembled monolayer and method of making.
Since their unveiling in 1992, mesoporous ceramics have inspired substantial interest, especially by adding self-assembling monolayer compounds to the surface(s) of the mesopores. By varying the terminal group of the self-assembling monolayer, various chemically functionalized materials have been prepared. A mesoporous material is defined as a material, usually catalytic material, having pores with a diameter or width range of 2 nanometers to 0.05 micrometers.
Exemplary of use of self-assembling monolayer(s) on a mesoporous material is the International Application Publication WO 98/34723 (E-1479 CIP PCT). The self-assembling monolayer(s) is made up of a plurality of assembly molecules each having an attaching group. For attaching to silica, the attaching group may include a silicon atom with as many as four attachment sites, for example; siloxanes, silazanes, and chlorosilanes. Alternative attaching groups include metal phosphate, hydroxamic acid, carboxylate, thiol, amine and combinations thereof for attaching to a metal oxide; thiol, amine, and combinations thereof for attaching to a metal; and chlorosilane for attaching to a polymer. A carbon chain spacer or linker extends from the attaching group and has a functional group attached to the end opposite the attaching group.
Methods of attaching and constructing the self-assembling monolayer on a mesoporous material involve solution deposition chemistry in the presence of water. More specifically, as reported by Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science, 1997, 276, 923-926 (Feng et al, 1997); and Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165 (Liu et al., 1998), a synthetic protocol to prepare monolayers of MPTMS (mercaptopropyl trimethoxysilane) within the pores of MCM-41 involved a 1-hour hydration step, followed by a 6-hour silanation step in refluxing toluene. At this stage, the silane coverage is limited to approximately 3.6-4.0 silane molecules/nm2 (this surface density is not enhanced by either extending the reaction time or increasing silane concentration). Following the silanation with a 2-3 hour azeotropic distillation drives the equilibria through the removal of reaction by-products, and increases this surface density to 5.0-5.2 silanes/nm2. This surface density is representative of typical silane-based monolayers. The monolayer coated mesoporous product is then isolated by filtration, washed extensively and then dried for several days. In summary, the overall procedure takes about 10 hours of laboratory prep time and 1-10 days of drying time. The time is driven by the kinetics of getting the self-assembling molecules into the mesopores and getting the water and any other solvent out of the mesopores.
The product obtained exhibits a maximum of 40% of the monolayer silicon atoms fully crosslinked for maximizing monlayer stability. Ideally, 100% of the silicon atoms would be fully crosslinked. Full crosslinking is having three of the four bonding sites linked to another silicon atom via an oxygen atom, with the fourth linked to the functional group terminated hydrocarbon chain. However, the presence of xe2x80x9cdanglingxe2x80x9d hydroxyl groups (OHxe2x80x94) cannot be avoided in the solution method and it is the presence of the xe2x80x9cdanglingxe2x80x9d hydroxyl groups that interferes with the crosslinking of the monolayer, thus placing a practical upper limit on the number of silicon atoms that are fully crosslinked of 40%.
Thermal xe2x80x9ccuringxe2x80x9d of silane monolayers, wherein typical thermal curing (ca. 150xc2x0 C.), of a silane monolayer creates a terminal to internal silane ratio of 1:2 corresponding to about 60% to 65% of attaching molecules (silicon) fully crosslinked.
Hence, there remains a need for a mesoporous material having self-assembling monolayer thereon with a greater fraction of the assembly atoms fully crosslinked. There is also a need for greater surface density of silicon atoms, which may also be expressed as a greater surface density of monolayer coverage. Finally, there is a need for a method of making these materials that is less time consuming.
The main difficulty in functionalizing microporous materials may be attributed to diffusion of the organic molecules intoto the small pore channels. In the last few years, both post-silanization and in-situ deposition have been successfully applied to mesoporous materials, in which the pore diameter is usually larger than 2 nm. The mesoporous materials (usually synthesized using surfactant micelles as templates) have very uniform pore sizes. Because of their high surface area and the open pore channels; functionalized mesoporous materials have been investigated for many adsorption and catalysis applications. However due to the large pore size and the amorphous nature of the materials, these materials are not likely to find application as size selective catalysts.
A zeolite is any one of a family of hydrous aluminum silicate minerals, whose molecules enclose cations of sodium, potassium, calcium, strontium, or barium, or a corresponding synthetic compound, used chiefly as molecular filters and ion-exchange agents. Compared to the mesoporous materials, the diffusion of organic molecules in zeolites is severely hindered by the small pore size. Deposition of silanes on the exterior surface is therefore greatly favored over silanation of internal surfaces. Heretofore, it had been believed that introducing organic functional groups to the internal pore surfaces of commercial zeolites to produce size selective microporous catalysts could not be achieved due to the size of the pores.
According to the present invention, the previously known functional material having a self-assembled monolayer on a substrate has a plurality of assembly molecules each with an assembly atom with a plurality of bonding sites wherein a bonding fraction (or fraction) of fully bonded assembly atoms (fully crosslinked assembly atoms) with the plurality of bonding sites (the plurality of bonding sites bonded to an oxygen atom) exceeds a maximum compared to solution deposition, and maximum surface density of assembly molecules greater than for solution deposition. For example, with the assembly atom silicon, having 4 bonding sites, the bonding fraction maximum for solution deposition was 40% as deposited or about 60% to 65% (a terminal to internal silane ratio of about 1:2) after thermal curing, and maximum surface density of silane molecules was 5.2 silanes per square nanometer. Note that crosslinking fraction and surface density are separate parameters.
The method of the present invention is an improvement to the known method for making a self-assembled monolayer on a substrate, wherein instead of a liquid phase solution chemistry, the improvement is a supercritical phase chemistry.
The present invention has the advantages of greater fraction of bridging oxygen bonds, and greater surface density of assembly molecules resulting in a lower defect coating that enhances thermal and chemical stability or resistance. Further, hydrolysis and deposition is complete within 5 minutes, a surprising rate enhancement of more than two orders of magnitude. Not only are the hydrolysis and deposition considerably accelerated relative to standard solution methods, but the final drying phase has been completely eliminated by the use of a supercritical fluid as the reaction medium. The product emerges from the reaction chamber dry and ready to use. This represents considerable timesavings.
Water is a necessary reactant in the hydrolysis and condensation chemistry of alkylsilanes to form self-assembled monolayers onto ceramic oxide surfaces. It must be present in appropriate (stoichiometric) amounts; too little will result in incomplete deposition and crosslinking and too much will result in bulk solution phase polymer formation. Experience has shown that approximately 1013 water molecules per square meter of available surface area is optimum. This amounts to approximately 2 water molecules for each silane to be anchored.
It is also important that this water be intimately associated with the surface and not free in solution. By having the water in close proximity to the ceramic oxide surface, the silane hydrolysis/condensation chemistry can only take place on the surface, thereby favoring the desired monolayer deposition and avoiding solution phase polymerization (which leads to bulk amorphous polymer and blocked pores). This association is necessary to obtain any thin film morphology, and is critical to obtain clean monolayer formation.
In addition, the water associated with the ceramic oxide surface must be evenly spread out across the surface. This causes the hydrolysis chemistry to be uniformly spread out across the ceramic oxide surface, which reduces monolayer defect formation, while at the same time minimizing bulk polymerization.
By adding the water first, and allowing it to fully equilibrate with the ceramic oxide surface, Applicants are able to exploit the natural affinity that these ceramic oxides have for water and are thus able to insure that these important conditions are met.
Adding water separately to a solution of silanes will result in bulk solution phase polymerization competing with any possible monolayer deposition. This is counter-productive since it significantly depletes the amount of silane available to form the monolayer and in the case of a mesoporous substrate, the bulk amorphous polymer will plug and block the pore channels, reducing the available surface area and restricting interfacial access, thus eliminating the most desirable features of such a material.
In the liquid solution deposition of the prior art, a wastestream is produced as a mixture of water, methanol, toluene and small amounts of mercaptan that failed to be deposited. It is impractical to separate this mixture, and therefore the mixture is usually disposed of as hazardous waste. According to the present invention using a supercritical fluid for solution deposition, the only by-product of the reaction (hydrolysis) is an alcohol (e.g. methanol), which is easily separated from the supercritical fluid (which can be recovered for recycling). In fact, the alcohol is of sufficient purity to represent a potential feedstock that can be sold or recycled.
A further advantage of using a supercritical fluid as the reaction medium is the elimination of flammable solvents, and performing the reaction under completely non-flammable conditions, which can be a significant concern upon scale-up.